1. Field of the Invention
The present invention relates to magnetic resonance (MR) imaging and, more particularly, to volumetric MR imaging of flow, i.e., 3-D MR angiography.
2. Description of the Prior Art
Any nucleus which possesses a magnetic moment will attempt to align itself with the direction of an applied magnetic field. In doing so, however, the nucleus processes around this direction at a characteristic angular frequency (the Larmor frequency) which is dependent on the strength of the magnetic field and on specific properties of the nuclear species (the magnetogyric constant of the nucleus).
When a substance such as human tissue is subjected to a static uniform magnetic field (polarizing field B.sub.o), the individual magnetic moments of paramagnetic nuclei in the tissue, in their attempt to align with this field, process about it in random order at their characteristic Larmor frequency. The net magnetic moment M.sub.O includes a component M.sub.L produced in the direction (z) of the polarizing field, however, the randomly oriented components M.sub.T in a plane which is transverse to the plane of the polarizing field (i.e., the x-y plane) cancel one another. If, however, the substance, or tissue, is irradiated with an additional magnetic field (RF excitation field B1), which is in the x-y plane and which is near the Larmor frequency, the longitudinal component of the magnetic moment M.sub.L can be rotated (flipped) into the x-y plane to produce a net transverse magnetic moment M.sub.T which is rotating in the x-y plane at the Larmor frequency. The degree to which the M.sub.L component is rotated into an component is referred to as the flip angle. The magnitude and the direction of the net magnetic moment (M=M.sub.L +M.sub.T) depends primarily on the amplitude and duration of the applied RF excitation field B1.
One practical value of this gyromagnetic phenomena resides in the radio frequency signal which is emitted as a result of the net transverse magnetic moment M.sub.T. A commonly used technique, referred to in the art as a "pulsed NMR measurement", applies the excitation field B1 for a short interval, and then receives the signal that is produced by the resulting transverse magnetic moment M.sub.T. Such pulsed NMR measurement cycles may be repeated many times to make the same measurement at different locations in the subject or to make different measurements using any of a number of preparative excitation techniques.
In MR imaging, magnetic field gradients are used in addition to the static magnetic field. Since the resonant frequency of the processing nuclei depends upon the precise magnetic field strength imposed upon it, the applied field gradients are used to provide a mechanism which encodes spatial information into the sensed frequency response signals. MR imaging devices correlate signal intensity at a given frequency with nucleus concentration and relaxation parameters at a given location. This provides spatial information which is used to make a map or image of the object, based upon signal intensity variations due to the concentration and/or relaxation time differences. Such techniques are now routinely used to form MR medical diagnostic devices which provide two-dimensional (2-D) images. Many pulse and gradient sequence variations are also known which can be used to enhance the diagnostic value of the images produced.
Until recently, the application of 3-D visualization methods in MR diagnostic imaging was rather tedious. Traditional 2-D acquisition techniques were not able to produce quality sufficient for 3-D display in a time frame which can be tolerated by a patient. Recent developments, such as described in my paper entitled "IMPROVEMENT OF 3-D ACQUISITION AND VISUALIZATION IN MRI" published in Magnetic Resonance Imaging, Vol. 9, No. 3, 1991, Pages 001-013 describes improvements in 3-D techniques which make 3-D MR imaging more attractive for medical diagnostic use.
One emerging MR imaging modality is flow or vascular imaging. One basic physical principle which is used to acquire flow images in the MR environment uses time-of-flight effects. Time-of-flight effects are based on the macroscopic motion of nuclear spins with longitudinal magnetization. Typically, the magnetization of a bolus of blood is labelled at one time (via a selective RF excitation) and detected at a later time (at readout). Because the bolus changes location between labelling and detection, the name time-of-flight is used. Inflow enhancement is a special case of time-of-flight, in which the bolus is excited and detected in the same slice. An example of inflow enhancement effects in MR imaging is shown in FIG. 1a, wherein the flow is perpendicular to the slice plane. During a single repetition time, some of the spins flow out of the slice and are replaced by others flowing into the slice. Repeated MR imaging sequences result in the stationary tissue becoming partially or fully saturated after a few repetition times, thereby diminishing any signal from the stationary tissue. The blood flowing into the selected slice provides unsaturated spins which produce high signal and high contrast relative to that of the adjacent stationary tissue. Thus, if the excitation pulse repetition time (TR) is short relative to the time required for the longitudinal magnetization to become substantially relaxed (T1), over the course of several TR intervals signal from stationary tissue will be attenuated as their spins become more and more saturated. Unfortunately, the flowing blood will also become partially saturated, which saturation manifests itself as a reduction in signal. However, some partially saturated spins in the blood will flow out of the slice and are replaced by unsaturated spins which have higher signal capability. As more unsaturated spins flow into the slice during each TR, more signal is available. The flow signal is maximized when all the partially saturated flowing spins in the slice are replaced each TR. Thus, signal enhancement is directly related to the flow velocity and slice thickness. Further information relating to time-of-flight effects can be found in the article "Time-of-Flight MR Flow Imaging: Selective Saturation Recovery with Gradient Refocusing", published in Radiology, 1986, No. 160, Pages 781-785.
Inflow enhancement also occurs in 3-D volume imaging. 3-D imaging sequences primarily differ from 2-D sequences in that a second spatial encoding gradient is used in combination with the RF excitation pulse to divide the imaging volume into N partitions. In 3-D imaging however, large volumes with many phase encoded partitions (N) results in saturation of flowing spins in the same manner as described above in 2-D imaging if velocities are low. As spins become progressively more saturated, they produce less transverse magnetization (M.sub.T), and consequently less signal, so enhancement will not be seen throughout the entire axial length of the volume, as shown in FIG. 1b. An illustration of the progressive saturation effects upon the net transverse magnetization is shown in FIG. 2 for flowing material at the flow entrance plane of the volume and again at the flow exit plane. The rate of signal attenuation depends primarily upon the repetition time TR, flip angle .alpha., and the longitudinal relaxation time T1.
Although various techniques such as Gradient Motion Rephasing (GMR), pulse sequences such as FISP (Fast Imaging with Steady Precession), FLASH (Fast Low Angle Shot), and MP RAGE (Magnetization Prepared Rapid Gradient Echo) and other techniques such as described in my above-mentioned paper, as well as my article entitled "INTRACRANIAL CIRCULATION: PULSE-SEQUENCE CONSIDERATIONS IN 3-DIMENSIONAL (VOLUME) MR ANGIOGRAPHY" published in Radiology 1989, Vol. 171, June 1989, Pages 785-791, are available, improvement is still desirable in order to overcome the problem illustrated in FIGS. 1b and 2. With existing MR angiographic techniques, a compromise between volume size, vessel contrast and scan time is made. As an example, the slab thickness must be relatively thin to avoid excessive spin saturation. In order to cover a sufficiently large area of the vasculature, the measurement has to be repeated at different positions, which undesirably increases the overall scan time.
The present invention is directed to reducing the effect of spin saturation which occurs when spins are repeatedly imaged while moving through a selected volume, without increasing the scan time and at the same time minimizing the requirement to modify existing hardware and software.